Module Name: src
Committed By: ad
Date: Wed Dec 11 20:50:32 UTC 2019
Modified Files:
src/sys/kern: vfs_bio.c
Log Message:
Add a comment.
To generate a diff of this commit:
cvs rdiff -u -r1.282 -r1.283 src/sys/kern/vfs_bio.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/kern/vfs_bio.c
diff -u src/sys/kern/vfs_bio.c:1.282 src/sys/kern/vfs_bio.c:1.283
--- src/sys/kern/vfs_bio.c:1.282 Sun Dec 8 20:35:23 2019
+++ src/sys/kern/vfs_bio.c Wed Dec 11 20:50:32 2019
@@ -1,4 +1,4 @@
-/* $NetBSD: vfs_bio.c,v 1.282 2019/12/08 20:35:23 ad Exp $ */
+/* $NetBSD: vfs_bio.c,v 1.283 2019/12/11 20:50:32 ad Exp $ */
/*-
* Copyright (c) 2007, 2008, 2009, 2019 The NetBSD Foundation, Inc.
@@ -123,7 +123,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: vfs_bio.c,v 1.282 2019/12/08 20:35:23 ad Exp $");
+__KERNEL_RCSID(0, "$NetBSD: vfs_bio.c,v 1.283 2019/12/11 20:50:32 ad Exp $");
#ifdef _KERNEL_OPT
#include "opt_bufcache.h"
@@ -1145,6 +1145,16 @@ already_queued:
/* Unlock the buffer. */
CLR(bp->b_cflags, BC_AGE|BC_BUSY|BC_NOCACHE);
CLR(bp->b_flags, B_ASYNC);
+
+ /*
+ * Wake only the highest priority waiter on the lock, in order to
+ * prevent a thundering herd: many LWPs simultaneously awakening and
+ * competing for the buffer's lock. Testing in 2019 revealed this
+ * to reduce contention on bufcache_lock tenfold during a kernel
+ * compile. Elsewhere, when the buffer is changing identity, being
+ * disposed of, or moving from one list to another, we wake all lock
+ * requestors.
+ */
cv_signal(&bp->b_busy);
if (bp->b_bufsize <= 0)
